This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-160215, filed Jun. 7, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a cathode-ray tube banded with a metal implosion-proof band using a shrink-fit method, and a method for manufacturing the cathode-ray tube.
A generally-used color cathode-ray tube includes a vacuum envelope which has a substantially rectangular glass-made panel, a funnel coupled to the panel, and a cylindrical neck connected to a small-diameter section of the funnel. The panel has on its inner surface a phosphor screen which includes a plurality of red-, green- and-blue-emitting phosphor layers and light-shielding layers. A deflection yoke is mounted on the outer surfaces of the neck and funnel so as to extend from the former to the latter. In the neck is arranged an electron gun for emitting a plurality of electron beams corresponding to the luminescent colors of the phosphor layers.
A shadow mask having a color selecting function is provided inside the panel between the electron gun and the phosphor screen. The shadow mask shapes electron beams emitted from the electron gun and projects beam spots on the phosphor layers of specified colors. A skirt portion of the panel is banded with a metal implosion-proof band in order to maintain implosion-proof characteristics of the cathode-ray tube.
In the foregoing color cathode-ray tube, the implosion-proof band is usually formed in a rectangular shape by a mild steel having a uniform width and thickness and thus a yield point is fixed all over the band. The implosion-proof band described above is generally banded against the outer surface of the skirt portion of the panel by the shrink-fit method. According to the shrink-fit method, the implosion-proof band, whose inner circumference is slightly shorter than the outer circumference of the skirt portion of the panel, is expanded by preheating, and the expanded band is fitted on the outer surface of the skirt portion and then cooled, thereby banding the skirt portion with the implosion-proof band. A high banding force by the implosion-proof band is obtained at room temperature, and the implosion-proof strength of the cathode-ray tube is secured.
The implosion-proof band is usually heated (450xc2x0 C. to 600xc2x0 C.) almost uniformly by a gas burner or high-frequency heating. In this case, the banding force (yield stress) of the implosion-proof band becomes substantially uniform on both long and short sides of the skirt portion.
In the case of a cathode-ray tube using a rectangular panel, when an envelope is evacuated in the manufacture stage of the cathode-ray tube, the central part of a panel is deformed concavely toward the neck. Consequently a tensile stress caused on the skirt portion varies from side to side, such as 5.0 Mpa on the long sides and 5.4 MPa on the short sides. Even though the cathode-ray tube is banded with an implosion-proof band formed of a mild steel having a uniform width and thickness, neither the long nor short side of the skirt portion is banded by the optimum banding force.
According to the implosion-proof standards (UL/CSA, etc.) employed in the field of cathode-ray tubes, the safety of a cathode-ray tube is judged from the number of glass fragments flying ahead of the cathode-ray tube when a metal ball is caused to collide with the front of a panel to implode a vacuum envelope. The number of glass fragments depends upon the design (strength) of the panel and funnel. By thickening the glass of the envelope, the envelope can be broken safely or prevented from being broken. However, in this case, there occurs another problem that the cathode-ray tube increases in weight.
As a measure against the above, a safe tube can be designed and produced by optimizing the banding force of an implosion-proof band. However, as described above, tensile stresses of the tube are different at long-side, short-side and corner portions of the skirt portion and so are the banding forces required for the respective portions.
In order to resolve the above problem, Jpn. Pat. Appln. KOKAI Publication No. 10-199452 proposes a cathode-ray tube wherein the short- and long-side portions of an implosion-proof band are caused to differ in cross-sectional area to increase the banding force of the long-side portions. Further, Jpn. Pat. Appln. KOKOKU Publication No. 7-21999 proposes a cathode-ray tube in which an implosion-proof band has a bent portion covering all the edge of the front of a panel and the bent portion is formed widely from the corner portions toward the central portion to increase the banding force of the long-side portions of the panel.
Using the foregoing implosion-proof band, the implosion-proof effect can be enhanced even though the radius of curvature of a panel is large. However, the scrap rates of materials for the band are high, which means an increase in waste. Moreover, a plurality of implosion-proof bands cannot be formed using common materials if their panels are slightly different in radius of curvature, thus causing a problem of a decrease in manufacturing efficiency.
The present invention has been developed in consideration of the above situation and its object is to provide a cathode-ray tube in which an implosion-proof band mounted on the outer surface of a skirt portion of a rectangular panel using a shrink-fit method is improved in structure and implosion-proof characteristics.
In order to attain the above object, a cathode-ray tube according to one aspect of the present invention comprises a vacuum envelope including a rectangular panel having a skirt portion; a phosphor screen formed on an inner surface of the panel; an electron gun arranged in the vacuum envelope, for emitting an electron beam to the phosphor screen; and a substantially rectangular implosion-proof band having a pair of first side portions opposed to each other and a pair of second side portions crossing the first side portions and banded against an outer surface of the skirt portion, at least part of each of the first side portions, of the implosion-proof band having a yield point which is different from that of another part of the implosion-proof band.
According to the cathode-ray tube having the above structure, the banding force of the implosion-proof band against the skirt portion can be optimized by causing the yield points of the first and second side portions of the band to differ from each other. The implosion-proof characteristics of the band can thus be improved by absorbing a difference between tensile stresses caused in the periphery of the panel.
According to the cathode-ray tube of the present invention, the skirt portion of the panel includes a pair of first side-walls contacting the first side portions of the implosion-proof band and a pair of second side-walls contacting the second side portions thereof, and the first side-walls and the second side-walls are applied with different tensile stresses. The yield point of the at least part of each first side portion of the implosion-proof band and that of another part of the implosion-proof band differ from each other in accordance with a difference in tensile stress between the first and second side-walls of the skirt portion.
For instance, when the tensile stress of the first side-walls of the skirt portion is greater than that of the second side-walls thereof, the yield point of a middle portion of each first side portion of the implosion-proof band is set greater than that of a middle portion of: each second side portion thereof.
For this reason, the banding force of the implosion-proof band against the first and second side-walls of the rectangular panel can be optimized, and the implosion-proof characteristics can be improved by absorbing a difference between tensile stresses caused in the periphery of the panel.
A method for manufacturing a cathode-ray tube according to another aspect of the present invention, comprises the steps of preparing a vacuum envelope including a panel with a skirt portion standing on a peripheral portion of the panel; preparing an implosion-proof band formed of metal and shaped like a rectangular frame, the implosion-proof band having a pair of first side portions opposed to each other and a pair of second side portions opposed to each other; expanding the implosion-proof band by heating such that at least part of each of the first side portions and another part of the implosion-proof band are heated at different temperatures to cause yield points of the at least part and the another part to differ from each other; and mounting the implosion-proof band expanded by heating on an outer surface of the skirt portion, then cooling the implosion-proof band, and banding the outer surface of the skirt portion with the implosion-proof band.
According to the manufacturing method of the present invention, that portion of the implosion-proof band where the yield point is heightened, is heated at temperature which is higher than another part of the implosion-proof band.
When the implosion-proof band is expanded by heating as described above, the yield points of respective portions of the band can be caused to differ from each other by heating the portions at different temperatures. Thus, the banding force of the implosion-proof band against the panel can be optimized, and a cathode-ray tube, which is excellent in implosion-proof characteristics, can easily be manufactured. At the same time, the implosion-proof band need not be changed in shape or provided with a bent portion in order to control the banding force. The implosion-proof band can thus be improved in versatility to lower the costs for manufacturing the cathode-ray tube.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.